The present invention relates to multilayered boron nitride/silicon nitride (BN/Si3N4) coatings and to a method and a system for forming such coatings.
Composite coatings are used in a number of gas turbine engine applications. It is important for these coatings to demonstrate resistance to recession and high temperature oxidation stability.
One of the problems with ceramic matrix composites is the instability of the fiber matrix interface to oxidation. The use of boron nitride as a replacement for carbon has showed an improvement in resistance to high-temperature oxidation. However, it is common knowledge that the oxidation resistance of CVD boron nitride fiber coatings in the presence of moisture is considerably lower. CVD boron nitride fiber coatings have been known to degrade at room temperature in the presence of moisture.
There has been an investigation on the effect of alternating layers of SiC and carbon in SiC/SiC composites. It has been found that in such a system the room temperature mechanical properties are similar to a SiC/SiC composite fabricated with a normal carbon interface. However, when the composite is prestrained beyond the proportional limit (so that the matrix was micro-cracked) and subjected to high temperature oxidation, the composite exhibits brittle mechanical properties. This has been attributed to the removal of the carbon interface by oxidation. In order to enable this type of system to work in elevated temperature applications, a material with a greater oxidation resistance than carbon has to be substituted.
Some speculated that boron nitride could be substituted for the carbon. The major problem with such a substitution however is in the processing of the material. Boron nitride is usually fabricated by reacting a boron halogen with ammonia. The deposition of SiC is affected by the presence of ammonia. Therefore, the deposition of boron nitride has to be carried out in a separate reactor. This results in extra heating/cooling cycles in the process and additional handling of the part from one reactor to the other. This makes a boron nitride/silicon carbide multilayering concept unattractive.